This disclosure relates generally to temperature sensing, and more particularly, to dual source self-calibration or auto-correction systems and methods for distributed temperature sensing.
Fiber optic Distributed Temperature Sensing (DTS) systems were developed in the 1980s to replace thermocouple and thermistor based temperature measurement systems. DTS technology is often based on Optical Time-Domain Reflectometry (OTDR) and utilizes techniques originally derived from telecommunications cable testing. Today DTS provides a cost-effective way of obtaining hundreds, or even thousands, of highly accurate, high-resolution temperature measurements, DTS systems today find widespread acceptance in industries such as oil and gas, electrical power, and process control.
The underlying principle involved in DTS-based measurements is the detection of spontaneous Raman back-scattering. A DTS system launches a primary light source pulse that gives rise to two back-scattered spectral components. A Stokes component that has a lower frequency and higher wavelength content than the launched light source pulse, and an anti-Stokes component that has a higher frequency and lower wavelength than the launched light source pulse. The anti-Stokes signal is usually an order of magnitude weaker than the Stokes signal (at room temperature) and it is temperature sensitive, whereas the Stokes signal is almost entirely temperature independent. Thus, the ratio of these two signals can be used to determine the temperature of the optical fiber at a particular point. The time of flight between the launch of the primary light source pulse and the detection of the back-scattered signal may be used to calculate the spatial location of the scattering event within the fiber.
One problem involved in the operation of DTS systems is proper calibration. DTS technology derives temperature information from two back-scattered signals that are in different wavelength bands. The shorter wavelength signal is the Raman anti-Stokes signal, the longer one is usually the Raman Stokes signal. After the light from the primary source at λ1 is launched in a temperature sensing fiber, the scattered power arising from different locations within the optical fiber contained in the backscattered Stokes and anti-Stokes bands travel back to the launch end and gets detected by single or multiple detectors. As the Stokes and anti-Stokes signals travel, they suffer different attenuation profiles a αStokes (aS) and αAnti-Stokes (aAS), respectively, due to the difference in the wavelength band for these two signals. For proper temperature measurement a correction needs to be made so that the two signals exhibit the same attenuation.
One approach that has been used is to assume that the attenuation profile is exponentially decaying as a function of distance. This creates an exponential function with an exponent called the Differential Attenuation Factor (DAF) that is multiplied by the Stokes signal to adjust the attenuation profile to that of the anti-Stokes signal. The ratio of the resulting two signals is then used to derive temperature. The DAF is the difference in attenuation (aAS-aS) between two different wavelengths.
The assumption of a smooth exponential decay however is not always a reality. A number of factors can cause the actual attenuation to deviate from the exponential form. Localized mechanical stress or strain, fiber crimping, chemical attack (eg. hydrogen ingression) all can induce abnormalities, and some of these can change with time. It has been recognized in the industry that some form of continuous calibration or auto-correction is needed to reduce all of these irregularities.
One successful approach for such continuous calibration or auto-s correction was described in U.S. Pat. No. 8,496,376 using a dual laser system to automatically correct for the differential attenuation that exists between the Raman Stokes and Anti-Stokes backscattered light. This is achieved by having the Rayleigh signal of one laser overlap with the Raman signal of the other laser. This approach can effectively negate the scattering effects that are not temperature dependent, resulting in a signal that only represents temperature effects. Such an approach requires however that the dual lasers must be fired alternately because the backscattered Rayleigh signal of each laser is much larger in size than the backscattered Raman signals, making it impossible to detect the backscattered Raman signals. This alternate firing of the lasers results in a reduced sampling rate for the system.
There is a need then for a much faster and more effective self-calibration or auto correction scheme.